The present invention relates to brakes used on, for example, commercial truck or trailer axles, and in particular to automatic slack adjusters which eliminate excess motion in a brake actuator mechanism used to apply the brake.
Over the life of the brake linings of a brake, such as a pneumatic drum brake used on commercial vehicle axles, as the brake's friction linings wear the clearance between the brake linings and their respective friction surfaces (for example, the inner surface of a brake drum) increases. This increasing clearance requires an ever-increasing range of motion from the brake actuator mechanism to move the brake linings from their rest position to the point at which the linings contact the friction surface.
It has become commonplace to include an automatic slack adjuster in the mechanical path between the brake actuator and the brake linings so as to eliminate excess lining travel slack as the brake linings wear. Such adjusters typically are: (i) located on a portion of a brake cam shaft which is outside of the brake (typically splined to the cam shaft); and (ii) coupled to a pushrod of a brake actuator such that when the brake actuator push rod is extended or retracted, the slack adjuster rotates about the longitudinal axis of the brake cam shaft. Thus, by extending or retracting the brake actuator pushrod, the slack adjuster causes the brake cam shaft to rotate about its longitudinal axis, which in turn rotates a brake actuation cam affixed to the end of the brake cam shaft located within the drum brake. The rotation of the cam either presses the brake linings into engagement with the brake drum inner friction surface or allows the brake linings to withdraw radially inward, away from the friction surface.
Automatic slack adjusters are typically designed to transmit brake actuator force to the brake cam shaft in the brake application direction with no relative motion between the adjuster and the brake cam shaft. When the brake actuation force is withdrawn, if there is greater than desired distance between the brake linings and the brake drum friction surface, the slack adjuster is permitted to rotate relative to the brake cam shaft an angular distance sufficient to remove some or all of this undesired slack, i.e., limiting the distance the brake linings withdraw from the brake drum friction surface so that the lining-drum clearance is maintained at a desired minimum.
In many automatic slack adjusters, a one-way clutch is used to accomplish the rotary adjusting movement, with a worm shaft located in the adjuster turning a worm gear (also known as a worm wheel; collectively, a “gear set” or “gear train”). The worm gear is coupled, typically via splines, to the brake cam shaft. When the brake actuator pushrod is retracted, the worm shaft rotates about its longitudinal axis, causing the worm shaft and worm gear to move relative to one another in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake cam shaft. As a result, when the brake actuator pushrod returns to its rest position the brake cam shaft does not return to its original rest position. Instead, the brake cam shaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. Because the rotation of the slack adjuster relative to the brake cam shaft results in reduction of brake lining clearance in the new rest position, the automatic slack adjuster compensates for brake lining wear.
Previously, automatic slack adjusters have had their one-way clutch mechanism, and in particular the gear which drives the one-way clutch, located adjacent to the worm shaft, in bores in the automatic slack adjuster housing. As shown in each of prior art FIGS. 1-3, corresponding respectively to FIG. 3 of U.S. Pat. No. 4,484,665, FIG. 1 of U.S. Pat. No. 5,327,999, and FIG. 1 of International Patent Application No. WO 03 083322, the one-way adjustment mechanisms (including, respectively, control worms 29, 15, and unlabeled worm engaging worm teeth 6) are located in their housings (respectively, 13, 1 and unlabeled cross-sectioned body) on the side of brake cam shaft (respectively, 4, 3, 3) which is in the brake actuation direction (illustrated by brake actuation direction arrows).
As shown by way of these examples of the prior art, the one-way clutch mechanisms have been located on the brake actuation direction side of the automatic slack adjuster housings for a number of reasons, including that this location avoids the relatively space-constrained region behind the automatic slack adjuster. Due to the location of various components such as brake actuator support brackets in this region, placing the adjustment mechanism on the brake actuation direction side of the slack adjuster allows the designer greater freedom to package the one-way adjustment device within the slack adjuster housing. The close quarters with other components in the vicinity of the automatic slack adjuster also has provided an incentive to locate the adjustment mechanism on the brake actuation direction side of the slack adjuster in order to ease access to the adjuster for in situ manual operation of the adjustment mechanism by a technician.
A significant problem with prior art automatic slack adjusters, however, is that during brake application the reaction force transmitted via the worm gear and worm shaft is focused in the same area of the housing where the adjustment mechanism is located. Thus, the forces of the brake application and the reaction forces generated by the worm gear set combine to apply very high stresses to the thin-section wall of the housing adjacent to the bore containing the gear drive of the one-way clutch adjustment mechanism and the bore through which the brake cam shaft passes (for example, in prior art FIG. 2, the thin wall section directly behind the intersection of the teeth of gears 12, 15). The high stresses in this thin wall region greatly reduces the durability of the prior art automatic slack adjusters.
Despite industry-wide awareness and attention to the high stresses on the housings and resulting durability problems, this issue has not been satisfactorily addressed prior to the present invention. For example, some have tried to improve durability by incrementally increasing the thickness of the thin wall portion of the slack adjuster housing, but this alternative has proven to be of limited utility because an increase in the size of the thin wall section necessitates a corresponding increase in the size of the spur gear which cooperates with the adjuster. The larger spur gear in turn requires a larger spur gear seat, which causes a concomitant increase in the local housing stresses which offsets improvements obtained from increasing the thickness of the thin-wall section. Further, as is recognized in the art, significantly increasing the size of the automatic slack adjuster housing is not a practical alternative due to the limited space envelope in the regions of the wheel and brake components at an axle end.
In view of the foregoing, it is an objective of the present invention to provide an improved automatic slack adjuster with superior durability. In addressing these and other objectives, the present invention provides a solution to the problems of the prior art by moving the thin-section wall to a region behind the brake cam shaft and worm gear, i.e., to the side of the automatic slack adjuster housing which is way from the brake actuation direction. In an unexpected development, we determined that this arrangement of the one-way adjustment mechanism components results in an automatic slack adjuster design which surprising leads to far lower stress levels in the thin-wall regions of the automatic slack adjuster that experienced in the thin-section wall regions of prior art automatic slack adjusters.
In the present invention, thin-wall region associated with the adjustment mechanism is now located outside of the load path through which the majority of the brake application and reaction forces pass from the brake actuator pushrod to the brake cam shaft. Because the thin-wall region of the housing is free of the majority of the brake application stresses, this region is subjected to lower local stress levels. Surprisingly, the reduction of stress levels in the thin-walled region can be dramatic. For example, initial finite element stress calculations have indicated that thin-wall region stresses in the present invention's automatic slack adjuster arrangement may be less that one-third of the high stress levels in prior art automatic slack adjuster housing designs. The magnitude of the stress level decrease is far in excess of the incremental reductions previously observed in prior attempts to improve prior art slack adjusters, such as by the above-noted increase in the thickness of the thin-wall portion of the adjuster housing.
The greatly reduced localized stress level provided by the present invention provide corresponding greatly increased levels of durability. Alternatively, a portion of the increased load-bearing capacity of the present invention slack adjuster could be utilized by the designer to provide an automatic slack adjuster with a load capacity which is significantly higher that previously available in conventional automatic slack adjusters, without a corresponding increase in the automatic slack adjuster's size. Another alternative available to the designer would be use of the additional strength and durability margin to design a smaller and/or lighter slack adjuster, saving weight, cost and installation space while still providing sufficient durability for heavy-duty service, as in commercial vehicle brake service.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.